Patent GB-374978-A (GB)
Title: Improvements in or relating to thermionic amplifying systems
IPC: H03D7/14
Filed: 1931-03-23
Abstract: 374,978. Valve-amplifying circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Marconi House, Strand, and MATHIEU, G. A., 3, Mayfield Gardens, Shirehall Park, both in London. March 23, 1931, No. 8936. [Class 40 (v).] In a supersonic heterodyne receiver, particularly for use with short waves, the frequencychanging stage comprises a pair of valves with their grids electrostatically coupled in parallel to the local source and in push-pull to the signal-receiving circuit. As shown in Fig. 2 (Prov.), the grids of the two valves V1, V2 are connected through two variable condensers C7, C8 and their common point is connected through a condenser C13 (which may comprise telescoping tubes) to a coil coupled to the local source LO. Grid bias, supplied to the grids through resistances R1, R2 or potentiometers, is separately adjustable. The signal input circuit SI, which may be the output circuit of a preceding amplifier, is connected at opposite sides through condensers C3, C4 to the respective grids. Plate modulation may be prevented by providing shunt condensers C9, C10 and series chokes CH3, CH4 in the output circuit; whilst across the output leads is connected a symmetrical oscillatory circuit including coils L1, L2 and condensers C11, C12 earthed at its mid-point. Jacks are provided across the condensers C11, C12 for checking circuit adjustments. The circuit is tuned to the intermediate frequency, and resistances may be included in it to enable both side-bands to be transmitted. Alternatively, if one side-band only is to be transmitted, the circuit may be tuned to the mean frequency of that band and the resistances omitted. In a modification, the condensers C9, C10 and the chokes CH3, CH4 are omitted, and neutralizing condensers are connected between the plate of each valve and the grid of the other. The input signals are fed through a transformer, the secondary of which is connected through variable condensers such as C3, C4 and through choke coils to the respective grids. The double condenser C7, C8 is preferably constructed as a differential condenser, Fig. 1 (Comp.), comprising two thimble-shaped electrodes AC7, ACS fixed to an insulating support and a rod-like electrode AC78 attached to a screw-operated spindle and adapted to be moved towards either of the fixed electrodes. Alternatively, the central electrode may be tubular and fixed on a central rod, and the outer electrodes may be independently slidable within the fixed electrode, and may after adjustment be secured by grub-screws to the rod. The coupling between the final highfrequency amplifiers V3, V4 and the frequencychanger may be made variable by providing coupling condensers K1, K2, Fig. 3 (Comp.). These preferably comprise parallel fixed plates situated on either side of apertures in the screen PSB between the amplifier screening box SB, and the frequency-changer box SB2. Centrally pivoted to the screen PSB is a disc CD, Fig. 4 (Comp.), which has two apertures arranged to register with the screen apertures. When the disc CD is rotated by a worm drive AH, the apertures are masked to an adjustable degree and the coupling condensers correspondingly reduced in capacity. Metallic iris diaphragms may replace the disc CD. When multistage high-frequency amplifiers precede the frequencychanging stage, amplification may be reduced without affecting selectivity by removing the valves of one or more stages and replacing them by coupling condensers, such as are described in Specification 375,891. The input circuit for the first high-frequency stage may comprise a symmetrical tuned circuit, Fig. 5 (Comp.), with the input leads from the aerial tapped upon the coils L1, L2. The circuit is enclosed in a screening box SB with apertures W to permit magnetic coupling with the coils L3, L4. This coupling may be varied by iris or other diaphragms as in Figs. 3 (Comp.) and 4 (Comp.) above. Electrostatic coupling is prevented by metal gauze windows in the apertures W. The receiving set is preferably constructed in the manner described in Specification 375,891.